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Forward and inverse P- wave velocity models along an E -W 280-km-long wide-angle reflection/refraction profile at 21°N across the northeastern South China Sea margin and Manila subduction zone reveal a 10–12 km thick thinned continental crust (TCC) located west of the Manila transcurrent fault (MTF) and Manila slab. The TCC is divided in two parts separated by the intra-continental velocity difference boundary (VDB). West of the VDB, the TCC is located above a high velocity layer (HVL). East of the VDB, the TCC consists of three layers: an undefined body with a velocity inversion at its base above the upper and lower crusts. The VDB and MTF merge east of the Hengchun peninsula showing that the TCC located east of the VDB decreases in size and disappears to the north. The distribution of earthquakes (EQs) shows that ∼80 km of crust have subducted east of the MTF since the onset of Luzon arc collision with Eurasia at ∼7 Ma. All published tomographies suggest the subduction of oceanic crust (OC). However, due to crustal corrections, up to a maximum width of 40 km of TCC might eventually exist in the upper part of the Manila slab instead of OC. Two end-members with 40 km of TCC or not in the upper part of the Manila slab were restored at ∼7 Ma. Since that time, the north Luzon island moved ∼600 km in the northwest while part of this motion was absorbed near the Hengchun peninsula by shrinking of Eurasia TCC, resulting in a ∼ 20° clockwise rotation of the Manila slab and MTF. Small Luzon arc blocks started to rotate 10°-30° clockwise at ∼7 Ma below northern Taiwan, 3–4 Ma in the Coastal range and 1 Ma in Lutao and Lanyu islands and not south of the Hengchun peninsula. Thus, both the clockwise rotation of the individual Luzon arc blocks and of the Manila slab and MTF might explain the collision process of the Luzon arc with the Eurasia continent. • Thinned continental crust (TCC) west of the Manila transcurrent fault and Manila slab. • Two adjacent stripes separated by an intra-continental velocity difference boundary composed the TTC. • The left-lateral motion guides the driving force controlling the collision of Luzon arc and Eurasian TCC. [ABSTRACT FROM AUTHOR]

Vertical edges along subducted slabs have been recognized in the majority of subduction zones. Surprisingly, slab edges evolved into Subduction-Transform-Edge-Propagator (STEP) faults in only a few regions; the conditions under which STEPs form are special. It is relevant to constrain the conditions that facilitate STEP fault initiation because they leave a clear geological footprint in the overriding plate, whereas vertical tears generally do not. We therefore study a candidate region for STEP fault initiation in the western Hellenic Subduction Zone. We investigate the structure and seismicity of the shallow western Hellenic Subduction Zone using a recent full-waveform inversion model which both captures details of crustal and upper-mantle structure, yielding constraints in the depth interval from 10 to 200 km where lithosphere-mantle interactions have tectonic expressions. The western end of the Hellenic Subduction Zone is fragmented near the Kefalonia Transform Fault. We identify a separate Epirus lithospheric fragment that is roughly vertical below the southern Albanides. We also identify a new and major contrast within the lithospheric mantle of the Ionian ocean basin, which aligns with a gradient in free-air anomalies. In the overriding plate, the Kefalonia Transform Fault zone accommodates right lateral strike-slip deformation. We interpret this fault zone as a proto-STEP fault that formed simultaneously with Pliocene fragmentation of the Epirus fragment. Comparing the recent evolution of the NW Hellenic slab edge with currently active STEPs indicates that along-trench variations in convergence velocity are a prerequisite for STEP fault initiation. Such velocity variations may result from subduction of continental crust along part of the trench. Resistance to sea ward tear propagation by a mechanically strong subducting plate may prevent variations in convergence velocity to occur, and thus STEP fault initiation. The amount of time over which a velocity contrast persists will also be relevant for STEP fault initiation. Mechanical coupling between upper- and lower-plate, and the deformability of the upper-plate appear to also play a role in the initiation of the STEP fault once a slab is fragmented. • The western end of the Hellenic slab is fragmented near the Kefalonia Transform Fault (KTF). • The separate Epirus fragment is roughly vertical below the southern Albanides. • We interpret the KTF as a proto-STEP fault that initiated simultaneous with Pliocene fragmentation of the Epirus fragment. [ABSTRACT FROM AUTHOR]

• We present 3D seismic attenuation (Q) model of the Nazca Plate subduction zone. • Our model shows clear high- Q slab and low- Q continental plate structures. • Our model for the Brazilian Shield shows high- Q at 40–80 km depth in the backarc. • Along-arc slab Q variations indicate different slab hydration states. • Relatively high- Q at the mantle wedge corner indicates cold thermal state. Seismic attenuation provides key constraints on understanding the structure and dynamics of subduction-zone system. In this study, we provide the first three-dimensional P and S -wave attenuation models beneath the Nazca-South America subduction zone by inverting t * data from 397 local shallow and intermediate-depth earthquakes. The study area covers the southern part of the Peruvian flat-slab region (11°–15 °S) (where the subducting Nazca Ridge is migrating), and the Central Andean Plateau (∼13°–18 °S) (associated with the subducting Nazca Plate) and the Brazilian mantle lithosphere. We obtain five major features in our attenuation model with seismicity including (1) well-defined along-strike and along-dip Nazca slab structure in terms of low attenuation, (2) high attenuation associated with the passage of Nazca Ridge and present location of the oceanic ridges, (3) high attenuation in the backarc mantle wedge and continental crust, (4) clear image of Brazilian Shield in terms of low attenuation, underthrusting to the west, and (5) high attenuation related to the slab dehydration beneath the volcanic arc at 100 and 200 km depths. In particular, prominent low- Q zones in the continental crust and mantle wedge beneath the active volcanic arc reflect source zones of arc magmatism caused by fluids from the slab dehydration. The observed along-arc differences in slab Q estimates can be attributed to different hydration states between the flat and normal-dip slabs due to the subduction of the Nazca Ridge. [ABSTRACT FROM AUTHOR]